Histopathological studies were continued on the cellular reactions induced by a single intratracheal instillation of silica in F344 rats, using tissues from previous lifetime and serial sacrifice experiments with silica samples, Min-U-Sil 5 quartz (MQZ) and hydrofluoric-acid-etched MQZ. These yielded high incidences of peripheral lung carcinomas, derived from hyperplastic areas of alveolar type II epithelium adjacent to silicotic granulomas. Additional histogenetic studies were conducted by using special stains on new sections from paraffin blocks. We examined the immunohistochemical localization of TGF-~1 in the induced silicotic granulomas and in the adjacent alveolar type II hyperplastic or adenoma- toid lesions and in the derived tumors, which were mostly alveologenic adenocarcinomas. Polyclonal antibodies to synthetic peptides corresponding to the first 30 amino acids of TGF-~1, anti-LC(1-30) and anti-CC(1-30), gave, respectively, intracellular localization of mature TGF-beta1 in fibroblasts and macrophages at the growing edge of granulomas and in septa adjacent to hyperplastic alveolar epithelium, and extracellular localization in the connective tissue matrix, especially adjacent to hyperplastic alveolar epithelium; an antibody to a peptide corresponding to amino acids 266-287 of the TGF-beta1 precursor sequence was localized intracellularly in the hyperplastic alveolar type II epithelial cells and in their adenomatoid proliferations, adjacent to silicotic granulomas, but not in the malignant cells of carcinomas. These findings suggest a pathogenic role of TGF-~1 in the mesen- chymal/epithelial reactions to silica leading to carcinogenesis. Electron microscopic studies on the penetration and localization of silica in rat lungs were made possible by improved techniques. Molecular analyses on paraffin embedded rat lung tissues with silica-induced lesions, particularly alveologenic carcinoma, were carried out using polymerase chain reaction and single stranded conformational polymorphism analysis to detect mutations in the ras oncogene. This study will correlate in vivo findings with those obtained in silica-treated BALB/3T3 and FRLE cell lines.